Improve tests

[jpco]

This is just brainstorming.

More test coverage:

• newer trip.rc stuff that didn't get "inherited" into trip.es
• regression tests for already-fixed github bugs done in Add more test cases; mostly regression tests #200
• try to come up with tests for every builtin + primitive
• syntax precedence done to at least a basic degree in Add more test cases; mostly regression tests #200
• variable semantics, incl. multiple assignment, multiple reference, subscripting, lexical/dynamic binding, settors

More "types of" tests:

• a script or something to run tests with multiple sets/combinations of CFLAGS/LDFLAGS, like -fsigned-char/-funsigned-char, the various -Dfoos, -fsanitize, etc.
• "in-C" unit testing -- there's stuff that can't be easily gotten at from the shell level
  • string-formatting constants -- %jd, %F, etc.
  • the getenv/setenv/putenv/etc. hack
  • interactive-input-related functions (readline, etc.)
• better CircleCI integration; I know I left some gaps :)
• documentation/initial corpus/dictionary to make fuzzing easy and more productive Done to at least a basic level with Add some initial afl-fuzz files  #272
• tests for the "canonical extension" scripts

Cleanup/meta-improvements:

• trip.es is just a grab-bag, so finish breaking it up into per-"topic" files
• tidy up test.es (potentially split out junit.es into its own file?)
• some tests assume shell features work that other files test for; improve that story